DI token for CngxTimelineMarkerHost. Provided by <cngx-timeline>;
injected optionally by CngxTimelineItem, which falls back to its own
[cngxTimelineMarkerContent] projection when no timeline is above it.
A token rather than an injected parent class: the row must stay usable
standalone, and a concrete parent type would make the two mutually
dependent and block the decompose schematic.